1. Field of the Invention
The invention relates to a compact gas containment and supply apparatus, particularly one which is readily human portable
2. Discussion of Prior Art
To enhance portability of pressurised gas containment vessels there is a general requirement for high strength combined with relatively low weight. Overwinding around an internal shell is a well established technique both for strength and for weight reduction in the manufacture of cylindrical pressure vessels, such as gun barrels, gas cylinders and the like. Such structures when pressurized are subjected to hoop stresses that are significantly higher than the axial stresses, and the use of an overwinding designed to carry a large part of the hoop load allows the design of the base cylinder to be directed towards meeting only the axial stresses, with a considerable potential for saving in weight. Traditionally such windings were of high tensile strength metal wires. Recent developments in composite material technology have led to the use of composites consisting of fibre windings in a resin matrix.
Toroidal pressure vessels offer an alternative geometry to cylinders. Toroidal vessels comprised of a metal or composite inner toroidal casing overwound with wire or resin matrix fibre composite material are known and are described for example in UK Patent Application 2110566. These can offer some reduction of weight over unwound toroidal shell structures. In the case of composite winding, manufacturing can be complex as conventional winding equipment does not readily allow for the application of resin bonding during winding. It proves difficult to ensure complete wetting of fibre by matrix resin and incompletely wetted fibres constitute zones of weakness in conventional resin matrix fibre composite structures.
An object of the present invention is to provide a lightweight and compact gas containment and supply apparatus based on a toroidal pressure vessel having fibre overwinding with a reduced weight, and which mitigates some of the manufacturing difficulties encountered in toroidal structures overwound with resin matrix fibre composite material.
According to the invention, a gas containment and supply apparatus comprises a gas reservoir vessel capable of pressurised gas containment fitted with a gas supply aperture, supply means connectable to the gas supply aperture at a first end to provide for supply of the gas through a second end, and control means to control the rate of supply of the gas, wherein the gas reservoir is a toroidal pressure vessel comprising a metallic toroidal shell having wound on the surface thereof a tensile load bearing layer of high tensile strength non-metallic fibre, the fibre being aligned in a substantially meridianal direction on the toroidal shell.
Both the fibre winding and the metal shell are intended to be load bearing. As with simple cylinders, these structures are subjected to significantly higher stresses in the meridianal direction than in the direction perpendicular to the meridian xe2x80x9cringwisexe2x80x9d around the torus. The fibre is intended to bear a proportion of the meridianal load only, and is therefore wound in a substantially meridianal direction rather than diagonally round the torus as is the case for prior art composite layers such as described by UK Patent Application 2110566. The metal shell bears the remaining meridianal load and all the load perpendicular to the meridian. The use of winding to take part of the larger meridianal load allows the metal casing to be designed around lower loading parameters, and this produces a lighter vessel than would be possible using a metal construction alone.
The invention offers a compact pressurised gas reservoir which is lightweight and has a toroidal shape, both of which features result in enhanced portability. The toroidal geometry has a flatter profile since it has a smaller minor diameter than a cylinder of equal volume. The shape is thus particular suited to stowage where a flat profile is desired, or to carriage on the human back since it protrudes less behind the wearer in use. The toroidal shape is also advantageous for carriage on a human back as it fits back curvature more easily. The compact shape means that, although some form of harnessing to enable carriage of the tank by the operator will still be needed, this can generally be simpler, and hence lighter, than is needed for conventional cylindrical apparatus, and makes it possible to dispense with the back plate which is traditionally found necessary for at least the larger back mountable cylindrical gas bottles. The ability to dispense with the backplate is an additional factor in both the reduction of overall weight and the lessening of the distance behind the wearer by which the apparatus protrudes, both of which contribute to enhanced portability. The flatter profile of the torus shape also lends itself to being carried in a suitable bag or satchel which offers greater ease of portability whilst still providing the necessary mechanical constraint.
An additional advantage accruing from the toroidal shape lies in the ability for supply means connection to be made on the inside face of the torus affording some protection and reducing the possibility of its shearing off as a result of external impacts. For this purpose the supply aperture is preferably located on the inside face of the torus. The supply means may be permanently connected to the shell but for ease of storage and to allow replacement of gas vessels the gas supply aperture preferably includes means to effect releasable connection of the supply means and a closure valve to prevent release of gas with the supply means disconnected.
A particular advantage of using overwinding accrues from the build-up of thickness of the winding fibre on the inside of the torus. The overwinding is thus able to take a greater proportion of the meridianal load on the inside of the torus, which is the zone where the overall meridianal load is highest. This effect obviates the need for significant extra metal thickness in the higher loaded zones and as a consequence, a metal shell comprising a torus of substantially circular meridianal section and substantially uniform wall thickness gives close to optimum pressure containment performance with minimum redundant metal weight. Some simplification in manufacture results. However, it will be appreciated that a circular cross section is not essential to the effectiveness of the invention, and the invention can be applied to torus-shaped vessels of nonconstant curvature which have non-circular meridianal and/or transverse section where such a shape is more suited to the application of the invention.
Suitable materials for the winding include composites of polymeric, glass and carbon or ceramic fibres in a thermosetting or thermoplastic matrix. A thermosetting resin could be applied to the fibres as a prepreg prior to winding and cured after winding. A thermoplastic resin could be incorporated by using fine impregnated fibre bundles with sufficient flexibility to allow the winding operation, as thermoplastic fibres intermingled with the structural fibres, or as a thermoplastic powder attached to the structural fibres. Regardless of the method used to interlace the thermoplastic the composite will require subsequent consolidation under pressure at elevated temperature. In all cases the fibres are aligned around the meridian of the toroidal vessel.
However, since the invention employs fibres in the overwinding in a meridianal direction to carry loading in that direction only, it offers the additional possibility of dispensing with matrix altogether, or at least for the bulk of the load-bearing depth with only a surface layer applied for protection. In this matrix-free preferred aspect of the invention the absence of a matrix produces a weight saving compared with pressure vessels consisting of a shell overwound with a conventional fibre and matrix composite material and also obviates the requirement that the process must be compatible with consistent wetting of fibre matrix material during production, so that a simpler winding process can be used.
The starting point for fibre selection for this dry-wound matrix-free aspect of the invention is the group whose use will be familiar in thermosetting and thermoplastic resin matrix composite materials. The material used for the fibre winding requires high tensile strength. It must be a material which experiences little loss of strength through abrasion during winding or use and thus does not require matrix material for the abrasion resistance and protection it confers. Similarly, its strength must be only weakly dependent on fibre length (the so-called length/strength effect), so that the need for a matrix material to transfer load across broken filaments is minimal. These requirements tend to weigh against the use of glass fibres and carbon fibres in this aspect of the invention.
The above problems can be avoided by the use of high tensile strength organic polymeric fibres as such materials tend to be less susceptible to surface defects and exhibit a small length/strength dependence. Thus, they show a reduced tendency to lose strength as a result of abrasion damage. The role of the matrix in transferring load across broken filaments is therefore less important in composites employing this type of fibre. Thus, the tensile load bearing layer preferably comprises a layer of high tensile strength polymeric fibre, the fibre being aligned in a substantially meridianal direction on the toroidal shell and being free of any matrix material for at least a substantial part of its depth. Aramid fibres are particularly preferred for this purpose.
However, prestressed polymeric fibres tend to be susceptible to creep and stress relaxation, which can lead to them losing tension over service life and moving out of position. In conventional composites such movement is prevented by the presence of the matrix. For suitability in the present invention without a matrix the fibres must have creep and stress relaxation properties which are sufficiently low that the fibres can be practically pretensioned to a degree where they are able to retain sufficient tension over time to maintain position on the torus wall under all practical environmental exposure conditions.
Polymeric fibres also exhibit stress rupture; that is under a sufficiently high static load they will eventually fail. The time to such failure is dependent on stress and temperature and may be tens or hundreds of years. In relation to the present invention the stress rupture properties of the fibre must be such that the fibre tension arising from any necessary pretension in connection with overcoming creep problems together with the additional tension arising from the pressure loading can be accommodated without causing stress rupture failure for the lifetime of the vessel under all practical environmental exposure conditions.
There is thus a requirement that a xe2x80x9cwindowxe2x80x9d exists in the fibre properties in which sufficient initial pretension can be applied to the winding to avoid later movement arising from loss of tension due to creep without the pretension being so high as to cause stress rupture failure. The matrix-free winding in the preferred aspect of the invention exploits those high tensile strength polymeric fibres which have this window to dispense with the use of matrix material which the prior art requires as an essential feature of pressure vessels having a composite overwinding.
It has been found that aramid fibres possess such a window in their properties, and such fibres are therefore particularly suited to the matrix-free aspect of the invention. Carbon, glass and ceramic fibres possess larger windows, but their use is militated against by the problems outlined above in relation to abrasion resistance and the length/strength effect. Intermingled mixed fibres comprising one or more of these plus aramid, for example intermingled aramid and carbon fibres, offer a useful compromise. The aramid fibres shield the carbon from much of the abrasion that occurs during the winding process. During service, as stress relaxation and creep occur in the aramid fibres, load is gradually transferred to the carbon fibres. This is of value in designs in which the aramid fibre would be close to its stress rupture limit.
It is evident that the aperture in the toroidal shell cannot be overwound. For convenience of design the gas reservoir vessel may be provided with a zone of thickened inner casing without overwinding in the region of the gas supply aperture. However, a likely fabrication route for the toroidal shell is to weld together two curved gutters, and in such cases some structural problems can arise from intersecting welds where a thickened zone is welded into the vessel.
To overcome these difficulties an annular or partially-annular lug may be fitted over the aperture in the toroidal shell prior to overwinding, which lug comprises an external surface to receive a meridianally wound layer of fibre, a lateral aperture, and an air passage to provide a communication channel between the aperture in the toroidal shell and the lateral aperture. This configuration obviates the need to vary shell thickness in the vicinity of the gas supply aperture by allowing overwinding of fibre around essentially the entire surface of the toroidal shell.
The lug is preferably partially annular, with a crescent shaped section to minimise discontinuities at its edges. The lug is conveniently welded to the shell, preferably offset from the central plane of the torus to avoid intersection with the ringwise welds, which could give rise to potential weakness. External lubrication, for example with PTFE tape, is also desirable to avoid Kevlar fretting at the crescent tips.
An additional advantage of winding with a mixture of fibres is that by incorporating higher-modulus carbon fibres the stiffness of the winding can be increased, allowing the meridianal stiffness of the overwound zone (i.e. the product of Young""s modulus and thickness) can be approximately matched to that of the non-overwound zone, reducing stresses which might be generated by discontinuities in stiffness.
The matrix-free overwinding is preferably covered with a protective coating. This serves to compensate in part for the absence of environmental protection conferred by the matrix in conventional fibre composite windings, and in particular to protect the fibre from visible and ultraviolet radiation which can adversely affect fibre strength (particularly where the overwinding uses the preferred aramid fibres), to keep moisture out of the winding, and to provide protection from abrasion. The coating may at its simplest take the form of a protective elastomeric layer applied over the wound fibre, perhaps as a paint. Alternatively, an impermeable coat is applied over the wound fibre, and a further layer of fibre is wound over the coat to which an appropriate compatible resin is applied. Thus the winding presents the external characteristics of a conventional resin matrix composite but the bulk characteristics of the winding, and hence its substantive mechanical properties, remain in accordance with the dry-wound, matrix-free preferred aspect of the invention with the attendant advantages detailed herein.
The fibre winding tension requires careful control to ensure that it is high enough to avoid the overwind becoming slack and vulnerable to slipping as stress relaxation and creep occur in the fibre over time but not so high as to induce stress rupture of the fibre. Furthermore, the winding may be overtensioned so as to apply a compressive prestress to the metal shell, and thereby the pressure at which yield of the shell takes place can be raised.
The winding tension is preferably varied during winding to produce even load distribution in the finished product. As multiple layers of winding are laid down the outer layers will apply some compressive load not only to the metal shell but also to the inner fibre layers. If a constant winding tension is maintained and the overwinding is deep enough this can result in the inner layers losing tension so that when they come under pressure loading in service they are unable to accept their full share of the load. The solution is to reduce the winding tension as winding proceeds, so that tensile loading is evenly distributed throughout all layers of the overwound fibre in the fully wound vessel. However, for thin-walled vessels the need to vary the winding tension may be of minor importance.
Use of overwinding in accordance with the invention allows selection of the failure mode, so that the more benign mode can be chosen for a given pressure vessel application. With an excess of fibre overwinding, failure will occur by hoopwise rupture, that is, via a meridianal crack caused by stress generated perpendicular to the meridian. With a deficiency of fibre overwinding, hoop failure will occur first, that is, a crack perpendicular to the meridian caused by meridianal stress. In the latter case, it is possible to impose a further selection by incorporating a variation in torus wall thickness to create a zone of weakness.
For the metal shell, the desire for reduced weight with strength leads to a preference for use of aluminium and its alloys or, most preferably, titanium and titanium alloys, although steel and other metals could be used, especially in less weight-critical applications of the invention.
To ensure consistent gas supply the control means preferably includes a pressure regulator which is preferably a two-stage regulator.
A particular application of the invention is in the field of breathing apparatus with the pressurised gas reservoir vessel serving as a breathing gas (oxygen, O2/linert gas mix, air, etc.) vessel and a breathing mask and user operable demand valve connected to second end of the gas supply aperture. The toroidal shape is readily portable, and the design is compact and lightweight which are important considerations for this application of the invention. The protection offered by connecting the supply to a site on the inside of the ring is clearly of particular value in this embodiment of the invention.